Geoscience Reference
In-Depth Information
Vegetation and Plant Assemblages
Anyone who suf ers from hay fever dreads the spring, when the air is full
of pollen—the dust-sized gametes made by the male part of the l ower. All
vascular plants produce pollen in dif ering quantities, but not all plants
disperse it in the same manner. Some plants, using showy l owers, attract
insects that disperse the pollen as they l y from plant to plant, collecting
food. Others produce massive quantities of pollen and release it into the
wind, some of which eventually comes to rest where it can fertilize another
l ower. Successful as this strategy is for the plant species, the majority of the
pollen never makes it to other plants.
h e “let over” pollen has proven to be a tremendous resource for scientists
who are looking for clues to the past. A portion of the pollen carried by the
wind eventually ends up in lakes, marshes, and even the coastal ocean, where
it is incorporated into the accumulating sediments. Pollen has a naturally
resistant exine, an outer wall that protects the genetic information on the
inside needed for the job of fertilization. When pollen accumulates in the
right depositional environments, the exine remains well preserved and rec-
ognizable for long periods of time.
For scientists, pollen grains are like i ngerprints: they are unique to each
plant species or genus and, ideally, identii able by their shapes and surface
markings. Equally important, the types and proportions of pollen in the
sediments provide information about ancient plant assemblages and there-
fore about past climate conditions. As forests, grasslands, and deserts and
their various combinations of plants respond to changes in climate such as
temperature and precipitation, the changing mix of pollens produced paints
a picture of that climate. For instance, a pollen specialist would draw very
dif erent conclusions about a past climate from sediments containing pine
and spruce pollen as compared to those containing oak and sage pollen.
Wildi res
Charcoal is also found in sediments from lakes and the coastal ocean, pro-
viding information about the size and frequency of past forest i res. h ese
i res—though sometimes catastrophic—are a natural consequence of cli-
mate, particularly in the higher elevations of the American West.
Periods of prolonged drought typically create conditions that result in
larger and more frequent i res, which produce large amounts of charcoal that
